This invention relates to a motor-driven power steering apparatus and method for a vehicle in which a motor is operatively connected with steerable road wheels of the vehicle through a clutch for assisting the steering operation of the driver. More particularly, it relates to such an apparatus and a method as described in which the clutch is turned off to disconnect the motor from the steerable road wheels only when an abnormality in the power steering operation is detected when the vehicle is travelling at high speeds.
FIG. 4 is a schematic representation of a conventional motor-driven power steering apparatus as described in Japanese Patent Laid-Open No. 62-255273 or Japanese Patent Laid-Open No. 63-215461.
Referring to the figure, designated at reference numeral 1 is a steering wheel which is turned by the driver of a vehicle for steering operation. A steering shaft or column, generally designated at 2, serves to transmit torque imparted by the driver to the steering wheel 1 toward steerable road wheels (not shown) through a pinion and rack mechanism. The steering shaft 2 includes a first shaft section 2a which is connected at its upper end with the steering wheel 1, and a second shaft section 2b which is connected at its upper end via a universal joint 4a with the first shaft section 2a. A steering torque sensor 3 is mounted on the steering shaft 2 for sensing the steering torque T of the steering wheel 1. The pinion and rack mechanism includes a first pinion shaft 5 which is connected via a universal joint 4b with a lower end of the second shaft section 2b of the steering shaft 2, and a rack rod 6 which is connected at its opposite ends with the unillustrated steerable road wheels and which has a first threaded portion 6a engaged with the first pinion shaft 5, and a second threaded portion 6b. The elements 1 through 6 together constitute a steering system.
A vehicle speed sensor 10 senses the speed V of the vehicle at which the vehicle is travelling, and generates a corresponding output signal to a control unit 9. The control unit 9 includes a microcomputer for controlling a motor 13 in the form of a DC motor and a clutch 14 in the form of an electromagnetic clutch based on the steering torque T and the vehicle speed V as sensed by the steering torque sensor 3 and the vehicle speed sensor 10 so as to properly perform power assist for steering. A battery 11 mounted in the vehicle is directly connected to the control unit 9 for supplying power thereto, and it is also connected through a key switch 12 and the control unit 9 to the motor 13 and a clutch 14 to be described later for energizing them, the key switch 12 being connected between the battery 11 and the control unit 9. The motor 13 has an unillustrated rotating shaft which is connected via the clutch 14 with a worm shaft 15 which is in meshing engagement with a worm wheel 16. The worm wheel 16 is connected with a second pinion shaft 18 which is engaged with the second threaded portion 6b of the rack rod 6. The worm shaft 15 and the worm wheel 16 together constitute a speed reduction mechanism for transmitting the output torque of the motor 13 to the rack rod 6 at a reduced speed for power assist.
FIG. 5 is a block diagram showing the detailed construction of the control unit 9 of the conventional power steering apparatus of FIG. 4. The control unit 9 includes a motor control means 91 for calculating a motor current Io to be supplied to the motor 13 based on the steering torque T and the vehicle speed V and for driving the motor 13, and a clutch control means 92 for supplying a clutch control signal or current C to the electromagnetic clutch 14 to control it such that the clutch 14 is switched off into a disconnected state when the vehicle speed V is equal to and above a predetermined speed Vo (for instance 50 km/hr.), and switched on into a connected state when the vehicle speed V is below the predetermined speed Vo, and a motor current sensing means 93 in the form of a grounded resistor for sensing an actual motor current I flowing through the motor 13 and feeding data thereof back to the motor control means 91.
The motor control means 91, while being fed back with the motor current I sensed by the current sensing resistor 93, applies a voltage to the motor 13 such that a current corresponding to the calculated motor current Io is supplied from the battery 11 to the motor 13 via the control unit 9. When the clutch control means 92 determines a failure or abnormality in the power steering apparatus on the basis of signals output from the various sensors when the vehicle is travelling at low speeds less than the predetermined speed Vo, the clutch control signal or current C is made to a low level to turn off the clutch.
FIG. 6 is a graph showing the clutch control signal C and the calculated motor current Io. As shown at (a) in FIG. 6, the clutch control signal C is "on" or at a high level (e.g., indicative of a clutch current of about 1 ampere) when the vehicle speed V is lower than the predetermined vehicle speed Vo and it is "off" or at a low level (e.g., indicative of a clutch current of 0 ampere) when the vehicle speed V is the predetermined vehicle speed Vo or above.
As shown at (b) in FIG. 6, the calculated motor current level Io increases with the decreasing vehicle speed V and also with the increasing steering torque T when the vehicle speed V is lower than the predetermined vehicle speed Vo, and it is equal to zero when the vehicle speed V is the predetermined vehicle speed Vo or above. Usually, the steering wheel 1 is heavy to turn (i.e., a greater steering torque is required to turn the steering wheel 1) during low speed travel, whereas it is light (i.e., a smaller steering torque is required) during high speed travel irrespective of the road and other conditions. Accordingly, the auxiliary torque or power assist by the motor 13 should be increased with a decrease in the vehicle speed V.
The operation of the conventional motor-driven power steering apparatus shown in FIGS. 4 and 5 will now be described with reference to FIG. 6.
When the vehicle speed V is lower than the predetermined vehicle speed Vo during low speed travel, the clutch control means 92 in the control unit 9 makes the clutch control signal C to a high level to turn the electromagnetic clutch 14 on into a connected state. The motor control means 91 generates a motor current signal Io based on the vehicle speed V from the vehicle speed sensor 10 and the steering torque T from the torque sensor 3 thereby cause the motor 13 to generate a necessary torque for power steering.
More specifically, with a reduction in the vehicle speed V, the required steering torque T increases, so the motor current level Io corresponding to the output torque of the motor 13 is thus set to an increased level in accordance with the decreasing vehicle speed. When the required steering torque increases due to some cause such as a change in the road condition or the like, the motor current level Io is set to a value corresponding to the increased level of the steering torque T.
Thus, the amount of steering torque or force required of the driver can be held substantially constant irrespective of the driving condition or the like.
When a system failure or abnormality is detected during low speed travel, the clutch control means 92 makes the clutch control signal C to a low level to turn the electromagnetic clutch 14 off, so that the motor 13 is disconnected from the worm shaft 15 to ensure safety in steering. For example, a system failure or abnormality is determined in such a case as when the motor current level Io as calculated by the motor control means 91 or the motor current I as sensed through the current sensing resistor 93 is of an abnormally high value, or when the output of the torque sensor 3 shows that the steering wheel 1 has not been operated by the driver for an unusually long period of time.
During high speed travel in which the vehicle speed V is greater than the predetermined vehicle speed Vo, the clutch control means 92 makes the clutch control signal C to a low level to turn the electromagnetic clutch 14 off, thereby disconnecting the motor 13 from the worm shaft 15. At the same time, the motor control means 91 operates to stop or interrupt the power supply from the battery 11 to the motor 13 via the key switch 12, and hence the motor 13 is stopped. In this situation, even if a system failure such as an abnormality or malfunction in the motor 13, the sensors 3, 10 or the like takes place during high speed travel in which the electromagnetic clutch 14 is turned off to disconnect the motor 13 from the rack rod 6, there will be no trouble or problem in the manual steering operation of the driver.
Subsequently, when the vehicle speed V decreases below the predetermined value Vo, the clutch control signal C from the clutch control means 92 becomes high again to turn on the electromagnetic clutch 14, and the motor control means 91 controls the motor current I on the basis of the vehicle speed V and the steering torque T as noted above.
Generally, in the event of a failure of the control unit 9, an improper motor current level Io can be supplied to the motor 13, thus causing undesirable forced rotation of the steering wheel 1 without regard to the driver's steering effort. In addition, in the event of a mechanical failure of the motor 13 with the clutch 14 held in a connected state, it becomes difficult for the driver to turn the steering wheel 1, that is, the state of incapability of steering results. Although the driver can deal with these failures by turning the clutch off during low speed travel, it is extremely difficult to well cope with them during high speed travel, thus spoiling safety in driving as is well known in the art.
Accordingly, to avoid the above-described situations, when the vehicle speed V exceeds the predetermined vehicle speed Vo, the clutch control signal C is made to a low level to provide for fail-safe operation and hence safe driving.
To summarize, as described above, with the above-mentioned conventional motor-driven power steering apparatus, the electromagnetic clutch 14 is normally connected, but disconnected only when a system failure is determined during low speed driving, whereas it is always disconnected during high speed driving irrespective of any system failure.
Therefore, each time the vehicle speed V decreases below or increases above the predetermined vehicle speed Vo, the electromagnetic clutch 14 is operated to turn on and off. Such frequent on-off operations of the clutch 14 produce great sounds, thus resulting in increased noise and an uncomfortable ride. In addition, the durability of the electromagnetic clutch 14 is accordingly reduced. Further, upon the electromagnetic clutch 14 being turned on and off, the driver gripping the steering wheel 1 is frequently subject to undesirable steering shocks.